FIG. 1 shows an electronic ballast for a discharge lamp in the prior art. The ballast includes a DC (direct current) power supply circuit 1, a power conversion circuit 2, a load circuit 3, a drive circuit 4 and a frequency control circuit 5. The DC power supply circuit 1 is configured to concert AC (alternating current) voltage from an AC power source Vin into DC voltage V1. The power conversion circuit 2 includes an inverter circuit 20 and a resonant circuit 25. The circuit 2 is configured to convert the DC voltage from the DC power supply circuit 1 into AC voltage by the inverter circuit 20 to apply the AC voltage across a load circuit 3 including at least the discharge lamp via the resonant circuit 25. The load circuit 3 includes an induction coil 30 connected with the output of the power conversion circuit 2, and an electrodeless discharge lamp 31 in the proximity of the coil 30. In an example, as shown in FIG. 2, the load circuit 3 may include a fluorescent lamp 32 connected with the output of the circuit 2, and a capacitor 33 connected between both filaments of the lamp 32.
The drive circuit 4 is configured to adjust an operating frequency of the inverter circuit 20 in accordance with a control signal from the frequency control circuit 5. The frequency control circuit 5 is configured to substantially supply the control signal to the drive circuit 4 to sweep the operating frequency of the inverter circuit 20 from a start frequency to an end frequency and thereby to increase the AC voltage (i.e., load voltage) V3 applied across the load circuit 3. For example, the end frequency is set to a resonance frequency of the combination of the resonant circuit 25 and the load circuit 3 after ignition of the discharge lamp (i.e., the lamp is lit), while the start frequency is set to a higher frequency than the resonance frequency. Accordingly, the operation frequency of the inverter circuit 20 is swept from the start frequency to the end frequency, so that the load voltage is increased. Thus, by increasing the load voltage, the discharge lamp can be successfully started even under the influence of change of the load impedance.
However, in the construction that a discharge lamp is ignited by resonance, the resonance characteristics corresponding to the above-mentioned combination can be changed by various factors such as change in ambient temperature; dispersion and secular change of components; and a metal enclosure (a reflector, etc.) of luminaire including a ballast. Especially, if the discharge lamp is an electrodeless discharge lamp 31, the lamp 31 requires higher Q than a fluorescent lamp and accordingly the load impedance is remarkably changed by the proximity of the reflector 90 to the lamp 31 as shown in FIG. 4. The load voltage is also changed. That is, an inductance component caused by the reflector 90 is connected in parallel with the induction coil 30, and accordingly an induction current I90 flows through the inductance component by electromagnetic induction from the coil 30. Consequently, as shown in FIG. 3, the resonance characteristics in start and operation modes shift to high frequency side, namely from the resonance characteristic “without change in load impedance” to the resonance characteristic “with change in load impedance”. Thus, if the end frequency fe to be set to the resonance frequency in the operation mode shifts to a lower frequency than the resonance frequency, the discharge lamp can be turned off spontaneously (i.e., flame failure can occur) and accordingly it becomes difficult to maintain the lighting operation of the lamp.
As shown in FIG. 1, if a variable resistor VR is connected to the frequency control circuit 5, the end frequency fe can be adjusted with the resistor VR. Therefore, the end frequency fe can be adjusted to the resonance frequency in the resonance characteristic “with change in load impedance”. However, various luminaire types exist, and accordingly every luminaire requires manual adjustment of the end frequency, and manufacturing cost becomes higher.
The device disclosed in Japanese Patent Application Publication No. 2003-332090 published on Nov. 21, 2003 includes a means for changing a setting value (target value) of the output of a power amplifier in response to a frequency in connection with an output frequency of an oscillation means.